Device for raster-stereographic measurement of body surfaces

ABSTRACT

In a device for raster-stereographic measurement of body surfaces, preferably of irregularly shaped objects, in which a raster pattern having a plurality of raster lines is projected onto the surface according to the method of raster stereography by a raster projector and a line image which is distorted by the surface shape is recorded by means of a camera which forms a stereo base with the raster projector, the raster pattern having periodically recurring, particularly emphasized lines, where the order number of the lines can be deduced from the regular recurrence of these particularly emphasized lines, the raster pattern which is projected onto the surface contains locally modulated raster lines, wherein at least adjacent raster lines are clearly distinguished from one another.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The invention is directed to a device for raster-stereographicmeasurement of body surfaces, preferably of irregularly shaped objects.The invention is used in particular for measuring parts of the humanbody or other biological objects or irregularly shaped technicalcomponent parts.

b) Description of the Related Art

In conventional raster stereography, a geometrical pattern of lines ordots is projected onto the surface to be measured and is recorded from adifferent direction by a camera, preferably a video camera, to which isconnected a process computer. Two different principles are known forprojecting raster lines. In the first principle, the line raster isachieved by line-scanning the surface with a light beam (described, forexample, by A. R. TURNER-SMITH, Journal of Biomechanics 21 1988!,515-529). The second principle was conceived by FROBIN and HIERHOLZER(Photogrammetric Engineering and Remote Sensing 47 1981!, 1717-1724) andconsists in a method for surface measurement by means of a projectedgrid containing periodically recurring lines of large thickness for amodel reconstruction of the body.

A disadvantage in the method of raster stereography consists in that theentire pattern reflected by the surface being analyzed is recorded bythe camera in one individual image. Although this enables a shortrecording time, the resulting image can be so complicated as to requirea costly image analysis in order to coordinate or assign the imagestructures to the projected raster lines. This is indispensable for acorrect model reconstruction of the body surface. For this reason, thedensity of the projected pattern must also not be too high, although ahigher density would increase the theoretical resolution of the image.The maximum possible density depends on the surface shape, in particularon the depth modulation.

OBJECT AND SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to increase thepossible scanning density without impeding the image analysis.

In a device for raster-stereographic measurement of body surfaces,preferably of irregularly shaped objects, in which a raster patternhaving a plurality of raster lines is projected onto the surfaceaccording to the method of raster stereography by means of a rasterprojector and a line image which is distorted by the surface shape isrecorded by means of a camera which forms a stereo base with the rasterprojector, this raster pattern having periodically recurring,particularly emphasized lines, where the order number of the lines canbe deduced from the regular recurrence of these particularly emphasizedlines, the object according to the invention is met in that the rasterpattern which is projected on the surface contains locally modulatedraster lines, wherein at least adjacent raster lines are clearlydistinguished from one another. For this purpose, raster lines withperiodic structures of different shape are advantageously used in theraster pattern (e.g., a diapositive).

Images of harmonic oscillations, as well as sawtooth modulation imagesor, in a particularly advantageous manner, square-wave modulation imagesare preferably used for this purpose. The oscillation images can bevaried with respect to amplitude, frequency and/or phase position inorder to obtain various distinguishable raster lines. When the rasterlines have different phase positions relative to one another, the rasterpattern is to be aligned substantially parallel to the stereo base toenable accurate analysis of the raster images in the camera picture.Further, the raster lines can be modified additionally or in asupplementary manner with respect to the light wavelength and/or theline thickness.

Further, unmodulated (straight) lines are advantageously insertedbetween modulated raster lines. Raster lines having a differentmodulation in pieces of lines are also suitable.

In order to maintain a clear distinction between the raster lines (so asto be substantially distinguishable from one another also withsurface-induced distortion), the raster lines are advisably repeatedsequentially within the raster pattern. Such sequences of differentlines can be composed of an optional number of lines in principle.However, sequences of three raster lines with different modulation areadvantageous (and sufficient in most cases). Nevertheless, it isadvisable (for reasons of simplicity) that the sequences also containunmodulated raster lines. This is advantageous especially for a roughanalysis in which only one type of raster line is detected.

The sequence of raster lines is formed in a particularly simple andadvantageous manner by using raster lines in the form of square-wavemodulation images which differ from one another appreciably with respectto amplitude and/or phase. The use of a sequence of unmodulated rasterlines (square-wave oscillation with zero amplitude) and square-waveoscillation with π-shifted phase is particularly advantageous.

When the image analysis is carried out with the use of raster lines inthe form of square-wave or sawtooth-wave modulation oscillations, theraster pattern is advantageously oriented substantially parallel to thestereo base and the raster lines are advantageously orientedsubstantially vertically with respect to the stereo base. In this way,it is ensured that the amplitude jumps (within a so-called epipolarplane of the stereographic imaging) in the focal plane of the projectorand the focal plane of the camera remain parallel to one another at alltimes regardless of the complexity of the shape of the body surface.This substantially facilitates the image analysis.

The invention is based on the idea that the scanning density in rasterstereography can only be increased when the raster lines can also becorrelated in an unambiguous manner in critical regions (regions of highdepth modulation of the body surface in which cross-overs anddistortions in the raster lines appear in the camera image). Accordingto the invention, this is effected by means of an individualidentification at least for closely adjacent raster lines, butpreferably also for a plurality of successive raster lines within theraster pattern by means of suitable modulation of the raster lines whichcan be analyzed in a simple manner by an automatic image processingprocess.

It is possible with the device according to the invention to increasethe scanning density of the body surface (i.e., the line density of theraster pattern) and in so doing to ensure an unambiguous assignment ofthe lines in the camera image. The image analysis with respect to theline allocation can be automated in a simple manner in particular whensquare-wave modulation oscillations are used as raster lines.

The invention will be explained more fully in the following withreference to an embodiment example.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows an example for a raster pattern according to the invention;

FIG. 2 shows the raster-stereographic recording geometry using theexample of a square-wave-modulated raster line;

FIG. 3 shows a schematic view for illustrating the longitudinal cameraerror; and

FIG. 4 shows a schematic view for illustrating the lateral camera error.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an example for a raster pattern as a diapositive 1 withraster lines 11 to 19 in the form of square-wave modulationoscillations. Raster lines 11, 14, 17, etc. are not modulated (amplitude0). Raster lines 12, 15, 18, etc. are modulated at constant amplitude Aand frequency I/T, where T is the oscillation period, and with phaseφ=0. Raster lines 13, 16, 19, etc. are modulated at the same amplitudeand frequency, but with phase φ=π.

The unmodulated raster lines 11, 14, 17 . . . can be found by priormethods by searching for smooth line pieces with a minimum length >T/2.Since these raster lines 11 to 19--all other conditions remaining thesame--now have the three times the spacing, the risk of an incorrectimage analysis is correspondingly reduced. The modulated raster lines12, 15, 18 . . . and 13, 16, 19 . . . are not found in this part of theimage analysis since the smooth line pieces are too short.

The modulated raster lines can be identified with a high degree ofreliability when making use of the phase information. In the embodimentexample shown in FIG. 1, the jumps take place at the same locations

    x=n·T/2 (n=11, 12, 13 . . . )

Depending on the geometry of the recording arrangement, it is possibleto predict the location of these jumps in the camera image regardless ofthe shape and position of the measured surface. Accordingly, during theline analysis the line shape can be demodulated in a manner similar to aphase-sensitive rectification. In the example shown in FIG. 1, rasterlines with phase φ=0 can thus be unambiguously distinguished from thosewith phase φ=π. Each of these groups of lines again has three times thespacing compared with an unmodulated line raster so that the risk ofincorrect allocation is also correspondingly reduced for the modulatedraster lines.

FIG. 2 shows a schematic view of the recording geometry which ispreferably used for this method. The nodal point of the projector K_(p)and the nodal point of the camera K_(k) form the stereo base. Theimaging systems are shown in an inverted manner as is conventional. Inthe example shown here using raster lines 11 to 19 in the form ofsquare-wave modulation oscillations, the stereo base 4 is orientedparallel to the diapositive 1 and vertically to the selected individualraster line 12 (ideal geometry). The raster line 12 is projected ontothe surface 2 to be measured. It is imaged in the camera focal plane 3.

The jump locations (locations of constant phase in diapositive 1)mentioned above are projected onto the surface 2 by the projector lenssystem. They lie on a bundle of planes having the common straight lineG. The straight line G lies parallel to the diapositive 1 and intersectsnodal point K_(p). In the ideal geometry according to FIG. 2, thestraight line G coincides with the stereo base 4. The planes of theamplitude jumps then form so-called epipolar planes E. FIG. 2 shows anindividual epipolar plane E which is defined by G and by the surfacepoint P. The traces of the epipolar planes E in the camera focal plane 3are referred to as epipolar rays or epipolars (shown in dashed lines inFIG. 2). They form a bundle of straight lines with a common intersection(vanishing point F) in the camera focal plane 3. This vanishing point Fat the same time represents the intersection of the camera focal plane 3with the straight line G.

In the projector focal plane in which the diapositive 1 is located, theepipolar rays form a system of parallel lines. With respect to themodulation of raster line 12, they represent the locations of constantphase. Thus, the locations of constant phase, especially the jumplocations of raster line 12, are represented in the camera focal plane 3by the epipolars, namely regardless of the position and shape of theanalyzed surface 2. With small deviations from the ideal geometry,certain errors occur as will be discussed in the following.

The vanishing point F and the epipolars of the jump locations in thecamera focal plane 3 can be calculated from the geometry of thearrangement. For example, this can be determined by known methods ofphotogrammetric calibration (e.g., Photogrammetric Engineering andRemote Sensing 48 1982!, 67-72 and 215-220).

The image analysis is then effected in such a way that the smooth,unmodulated raster lines 11, 14, 17 are first determined by means of aline search algorithm. In a second step, those raster lines 12, 15, 18having a positive jump on the epipolars at constant phase φ=0, 2π, 4π,etc. and a negative jump at phase φ=π, 3π, 5π, etc are searched.Subsequently, the raster lines 13, 16, 19 having a jump in the oppositedirection, respectively, at these locations are searched.

A variant of the raster pattern 1 shown in FIG. 1 consists in modulatingall raster lines with different phases. The repetition frequency ofraster lines with the same phase (3 in the preceding example) canaccordingly be increased and the reliability of the line identificationcan be improved. However, the number of line codings enabled in thismanner is limited by the resolution of the camera.

In principle, it is also possible to make use of the amplitude forcoding. However, since the imaging of the amplitude varies depending onthe surface inclination, only rough gradations, as in the precedingexample, are practical. The period length T or (spatial) frequency canalso be used for coding, wherein the camera resolution is likewise thesubstantial limiting factor. In order to reduce the risk of aliasingeffects, the frequencies employed should have no harmonics of a commonbase frequency.

Further, raster lines having pieces with different modulations are alsoconceivable, although likewise under the restrictions mentioned above.

Finally, different waveforms can be used. The advantage of square-wavefunctions consists in simple local phase detection. Provided asufficient number of complete periods T can be measured, sine-shaped orsimilar constant waveforms are also possible. In this case, also, theraster lines are identified by filtering by means of phase-sensitiverectification.

Possible deviations from the ideal geometry assumed in the precedingwill be discussed in the following for the raster line structures withsquare-wave modulation which are applied in the example.

In practice, it is not possible to realize the ideal geometry exactly.Therefore, errors occur, as a result of which the locations of constantphase in the camera focal plane can no longer be predicted accurately(i.e., without knowledge of the position and shape of the surface).Therefore, for practical application, it is important to know at whatmagnitude possible alignment errors will have a negligible effect on theaccuracy of prediction of the epipolars.

The geometry errors involve the orientation of the stereo base 4relative to the diapositive 1. If the stereo base 4 is not alignedparallel with the diapositive 1 and vertically with respect to theraster lines 11 to 19, the camera nodal point K_(k) does not lie on thestraight line G (FIG. 2). In this case, the locations of constant phasein the camera image depend upon the position and shape of the surface.

However, this dependence is low if the depth modulation of the surfaceis small in comparison to its distance from the stereo base 4 and if thecamera nodal point K_(k) lies sufficiently close to the straight line G.The resulting errors can be estimated from the lateral deviation Δx andfrom the longitudinal deviation Δz of the camera nodal point K_(k) fromthe straight line G (FIGS. 3 and 4).

FIG. 3 shows the effect of a longitudinal deviation Δz. The diapositive1 (not shown in the drawing) and the straight line G are at right anglesto the drawing plane. The epipolar planes E (surfaces of constant phase)are therefore also at right angles to the drawing plane and pass throughK_(p). The camera nodal point K_(k) diverges from the reference positionon the straight line G by Δz. The image rays K_(p) P and K_(p) P'extending in an epipolar plane E strike the surface 2 at points P and P'at different depths z and z¹ (depth modulation δz of surface 2). SinceK_(k) does not lie on G, the camera image rays K_(k) P and K_(k) P' alsodo not lie on the epipolar plane E. Therefore, the surface points P andP' are no longer projected on the same epipolars in the camera focalplane 3. This gives an error

    δx=-δz·Δz·x·c.sub.k /z.sup.3 ( 2)

or

    δx=-δz/z·Δz/z·x/z·c.sub.k (2a)

in the camera image point with respect to the epipolars for thereference value of distance z, where x represents the lateral distanceof this point from the center plane (plane of symmetry of thearrangement) and c_(k) represents the calibrated focal length of thecamera (principal distance).

A lateral deviation Δx has a similar result (FIG. 4):

    δx=δz·Δx·c.sub.k /z.sup.2 (3)

    δx=δz/z·Δx/z·c.sub.k   (3a)

It will be seen from the form of equations (2a) and (3a) that the errorsdepend in each instance on the ratio of the deviations Δx and Δz to thedistance z, that is, the errors are generally small. The other factorsin the equations are, as a rule, smaller than 1. Moreover, the error isnaturally proportional to the principal distance c_(k).

Both errors are additive. Allowing for the finite resolution of thesemiconductor video cameras which are customarily used, the permissibleerrors Δx and Δz of the camera position generally lie in the order ofmagnitude which can be maintained at a low expenditure on alignmentmeans provided the depth modulation δz/z of the surface lies in theorder of magnitude of 10-20% of the camera-to-object distance.

Accordingly, the application of raster lines 11 to 19 with square-wavemodulation as discussed in the example results in a device withincreased raster line density which can be realized in a simple mannerand by which the image analysis of the raster imaging which is distortedby the surface can be effected easily without disproportionately highexpenditure on alignment means.

While the foregoing description and drawings represent the presentinvention, it will be obvious to those skilled in the art that variouschanges may be made therein without departing from the true spirit andscope of the present invention.

What is claimed is:
 1. In a device for raster-stereographic measurementof body surfaces in which a raster pattern having a plurality of rasterlines is projected onto a body surface according to the method ofraster-stereography by a raster projector and a line image which isdistorted by the surface shape is recorded by a camera which forms astereo base with the raster projector, said raster pattern havingperiodically recurring, particularly emphasized lines, where the ordernumber of the lines can be deduced from the regular recurrence of theseparticularly emphasized lines, the improvement comprising: means forproviding that the raster pattern which is projected onto a surfacecontains continuous locally modulated raster lines modulated in adirection perpendicular to a direction of extension of the respectiveraster lines, wherein at least adjacent raster lines are clearlydistinguished from one another.
 2. The device according to claim 1,wherein said raster pattern from said providing means contains rasterlines with periodic structures of different shape.
 3. The deviceaccording to claim 2, wherein said raster pattern from said providingmeans contains raster lines in the form of representations of harmonicoscillations.
 4. The device according to claim 2, wherein said rasterpattern from said providing means has raster lines in the form ofsquare-wave modulation oscillations.
 5. The device according to claim 2,wherein said raster pattern from said providing means has raster linesin the form of sawtooth modulation oscillations.
 6. The device accordingto claim 2, wherein oscillation images with different amplitude are usedas raster lines in said raster pattern from said providing means.
 7. Thedevice according to claim 2, wherein oscillation images with differentphase position are used as raster lines of said raster pattern from saidproviding means, wherein said raster pattern is arranged substantiallyparallel to a stereo base.
 8. The device according to claim 2, whereinoscillation images with different frequencies are used as raster linesin said raster pattern from said providing means.
 9. The deviceaccording to claim 1, wherein raster lines with different lightwavelengths are used in said raster pattern.
 10. The device according toclaim 1, wherein raster lines having pieces with different modulationare used in said raster pattern.
 11. The device according to claim 1,wherein unmodulated raster lines are also contained in said rasterpattern between differently modulated lines.
 12. The device according toclaim 1, wherein different raster lines are repeated sequentially insaid raster pattern.
 13. The device according to claim 12, wherein asequence of raster lines is composed of an optional number of differentraster lines.
 14. The device according to claim 11, wherein only thesame raster lines of different sequences, can be evaluated at first in arough analysis.
 15. The device according to claim 12, wherein saidraster pattern is composed of recurring sequences of raster lines in theform of square-wave modulation oscillations with different phaseposition (φ) and/or different amplitude (A).
 16. The device according toclaim 15, wherein said raster pattern is formed of sequences of anunmodulated raster line and two raster lines in the form of square-wavemodulation oscillations with π-shifted phase.
 17. The device accordingto claim 4, wherein said raster pattern is oriented substantiallyparallel to a stereo base, wherein the direction of the raster lines isoriented substantially at right angles to the stereo base.
 18. Thedevice according to claim 13, wherein said optional number is three. 19.The device according to claim 14, wherein the same raster lines ofdifferent sequences are unmodulated raster lines.
 20. In a device forraster-stereographic measurement of body surfaces in which a rasterpattern having a plurality of raster lines is projected onto a bodysurface according to the method of raster-stereography by a rasterprojector and a line image which is distorted by the surface shape isrecorded by a camera which forms a stereo base with the rasterprojector, said raster pattern having periodically recurring,particularly emphasized lines, where the order number of the lines canbe deduced from the regular recurrence of these particularly emphasizedlines, the improvement comprising: means for providing that the rasterpattern which is projected onto a surface contains raster lines withdifferent line thicknesses, wherein at least adjacent raster lines areclearly distinguished from one another.
 21. The device according toclaim 20 wherein said raster pattern from said providing means containsraster lines with periodic structures of different shape.
 22. The deviceaccording to claim 21 wherein said raster pattern from said providingmeans contains raster lines in the form of representations of harmonicoscillations.
 23. The device according to claim 21 wherein said rasterpattern from said providing means contains raster lines in the form ofsquare-wave modulation oscillations.
 24. The device according to claim21 wherein said raster pattern from said providing means contains rasterlines in the form of sawtooth modulation oscillations.
 25. The deviceaccording to claim 21 wherein oscillation images with differentamplitude are used as raster lines in said raster pattern from saidproviding means.
 26. The device according to claim 21 whereinoscillation images with different phase position are used as rasterlines of said raster pattern from said providing means, wherein saidraster pattern is arranged substantially parallel to a stereo base. 27.The device according to claim 21 wherein oscillation images withdifferent frequencies are used as raster lines in said raster patternfrom said providing means.
 28. The device according to claim 21 whereindifferent raster lines are repeated sequentially in said raster pattern.